I changed the catalog thumbnail to differentiate it from the original (I did it on every single swatch). The Myla Set was a Clothing Outfit, so I changed it to Clothing Bottom and clicked Apply to all Swatches. In the preview window, the top has successfully been removed. I didn't change the LOD option (I don't know what it does) it's on LOD 0. I duplicated the package, exported the mesh, edited it in Blender, imported the edited mesh. I can take screenshots if it helps you understand where the problem is. Today, I finally managed to do it but I don't know why, it won't show up in my game (I checked every single category, everyday, men, women etc.) I have no experience at all dealing with meshes, Blender, S4S etc. I really wanted the skirt (on its own) of the Myla set by Simpliacity so I decided to do it myself for my personal use.
